This application claims the benefit of Korean Patent Application No. 2002-73471, filed on Nov. 25, 2002, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a nano-actuator, and more particularly, to an electric switching device that uses a chalcogenide material as a switching medium, and an electric circuit device including the electric switching device.
2. Description of the Related Art
A micromachining technique generally makes it possible to manufacture low priced radio frequency (RF) devices with high performance. Microelectromechanical system (MEMS) RF devices have some advantages, such as, a very low isolation and insertion loss, a consumption of very small power, and a radio frequency exceeding THz. Also, the MEMS RF devices have an operating voltage of about 30 to 50V. If these MEMS RF devices adopts a switching capacitor, they obtain a performance lower than about 0.1 dB at a frequency of 40 GHz when using a low-loss dielectric film and a high conductive metal. A loss at a frequency equal to or greater than 20 Ghz is mainly due to a resistance (Ω) of a metal wiring. The resistance of a switch is usually about 0.25Ω, which is a reasonable value, and can be applied to a phase shifter. An MEMS phase shifter has a far lower loss than a p type-intrinsic-n type(PIN) diode phase shifter or a PIN transistor phase shifter. The loss of such a phase shifter is mainly an ohmic resistance loss.
Examples of a conventional RF switch include a capacitive membrane switch (a type of switching capacitor) or an ohmic contact switch. A shunt RF switch, which is a type of capacitive membrane switch, will be described with reference to FIGS. 1 and 2.
Referring to FIG. 1, a single first RF signal line 12 and a pair of second RF signal lines 14 are disposed in strips on a substrate 10. To be more specific, the first RF signal line 12 is disposed between the two second RF signal lines 14 such that they are spaced apart from one another. The two second RF signal lines 14 are coupled to each other by a beam membrane 16. The beam membrane 16 has the shape of a bridge and intersects the first and second RF signal lines 12 and 14 so that the beam membrane 16 is a predetermined distance above the first RF signal line 12. A portion of the first RF signal line 14 over which the beam membrane 16 crosses is coated with a dielectric film 18. The beam membrane 16 is a predetermined distance above the dielectric film 18. In this structure, an RF signal is applied to the first RF signal line 14. Reference numeral 20a denotes a path along which an RF signal is carried when no voltage is applied to the beam membrane 16.
When a direct current (DC) voltage is applied to the beam membrane 16, the beam membrane 16 descends toward the dielectric film 18 because of a difference in potential between the beam membrane 16 and the first RF signal line 12. Consequently, the beam membrane 16 comes into contact with the dielectric film 18. At this time, a metal-insulator-metal (MIM) capacitor is formed among the beam membrane 16, the dielectric film 18, and the first RF signal line 12, such that the RF signal passes through the first RF signal line 12 and discharges into the second RF signal lines 14, which are ground lines. Such a capacitor-typed RF switch provides an isolation of an RF signal that varies depending on the dielectric constant of the dielectric film 18. As the ratio of an on-state capacitance to an off-state capacitance increases, the characteristics of the signal isolation are improved. Hence, the switching speed of the RF switch and the RF signal isolation are improved by using an SBT (SrBi2Ta2O9) or BST((Ba1-xSrx)TiO3) film with a high dielectric constant as the dielectric film 18.
The durability of the capacitive membrane switch does not depend on its mechanical structure but is shortened due to charging of a dielectric film. In charging of a capacitor membrane switch, charges tunnel through the barrier of a dielectric film due to poole-Frankel emission that occurs at an electric field of 1 to 3 MV/cm. Accordingly, the tunneling charges badly affects an electric field that is necessary to operate the switch, or impedes a release of the switch, which may lead to a slow switching-off. A breakdown voltage of the dielectric film drops since charges trapped in the dielectric film screen an external electric field. Charges may degrade the characteristics of the dielectric film while recombining with each other during several seconds to several days. Such a possibility that the characteristics of the dielectric film of the capacitor membrane switch are degraded can be reduced by lowering an external voltage, that is, by lowering an operating voltage.
However, the driving of a capacitive membrane RF switch at a low voltage weakens the mechanical strength of components that support the RF switch. This creates an advantage of lowering a pull-down voltage, but may weaken the durability of the RF switch.
Also, the capacitor membrane RF switch operates at a switching speed of about 1 μs when a high DC voltage, for example, no less than 20V, is applied.
As described above, since the mechanical durability and pull-down voltage characteristics of membrane RF switches conflict with the speed thereof, an appropriate design of the membrane RF switches is difficult.